Generally, various types of sealing members are configured by rubber or a polytetrafluoroethylene resin (Teflon). In recent years, sealing members made of expanded graphite which is superior in terms of heat resistance and the like have widely been developed.
As shown in FIG. 12, such expanded graphite consists of a bellow-like structure 1 having a thickness H (about 5 to 10 mm) in which acid-treated graphite 1A having a thickness of HO and a laminate structure of graphite crystals 1a is subjected to a heat expansion process and as a result laminations are opened in the laminate direction (the direction indicated by arrow a) so that a gap G is formed between the graphite particles la as shown in FIG. 13.
A sheet-like sealing material made of expanded graphite such as a gasket sheet is produced by using such expanded graphite having the above-mentioned bellow-like structure, in the following manner. As shown in FIG. 14, an expanded graphite mat-like member 200 which is expanded by heating so as to have a thick mat-like shape and in which both the surface and inner layer portions have a uniform composition is supplied to a pair of upper and lower pressure rolls 201, 201 via a belt conveyor which is not shown, and passed therethrough, whereby the expanded graphite crystals 1a are pressurized to be integrated with each other so as to produce a sheet-like sealing material made of expanded graphite 202. During the production, the bellow-like expanded graphite 1 (see FIG. 13) which is at a low density and exists in the surface layer portion (this means both the front and back layer portions of the mat-like member 200, and hereinafter such portions are referred to as merely the surface layer portion) of the mat-like member 200 suffers strong compression and shearing forces when it passes through the pair of pressure rolls 201, 201. As shown in FIG. 15, therefore, the shape of the expanded graphite is changed into a thin-leaf compressed member 203 which has a large area and a small thickness of t1. By contrast, the bellow-like expanded graphite 1 existing in the inner layer portion of the mat-like member 200 suffers compression and shearing forces which are smaller in degree than those applied to the graphite of the surface layer portion, when it passes through the pair of pressure rolls 201, 201. As shown in FIG. 16, therefore, the shape of the expanded graphite is changed into a compressed member 204 which has a smaller area and a larger thickness of t2 as compared with the thin-leaf compressed member 203 of the surface layer portion.
FIG. 17 is a perspective view showing a part of the sheet-like sealing material made of expanded graphite 202 of the prior art which is produced as described above, in cutting away in a step-like manner. In the surface layer portion, the thin-leaf compressed members 203 such as shown in FIG. 15 are in a regular and high orientation state with high density so that the longitudinal direction of each member is perpendicular to that of the sheet. On the other hand, in the inner layer portion, the thicker compressed members 204 such as shown in FIG. 16 are arranged irregularly or randomly.
In the thus configured sealing material made of expanded graphite of the prior art, the mechanical strength is originally low and the base material is formed only by pressurizing expanded graphite, and the thin-leaf compressed members 203 are highly oriented with high density. When a tensile force is applied, therefore, the sealing material exhibits behaviors such as (1) cracks formed in the thin-leaf compressed members 203 of the surface layer portion grow, and (2) the surface layer portion in a high orientation state and with high density forms cracks when a small bending stress is applied. The cracks rapidly propagate to the inner layer portion, and the sealing material is finally broken. In this way, such a sealing material has an elongation of substantially zero and a very small tensile strength. When the sealing material made of expanded graphite is handled or various kinds of sealing members are worked, therefore, the materials must be carefully treated so that a bending or tensile force is not accidentally applied. Consequently, there results not only a problem in that products such as a sealing member using the sealing material made of expanded graphite have a narrow application range, but also a problem in that working means are naturally restricted and product productivity is low.
Furthermore, the sealing material made of expanded graphite of the prior art is so poor in flexibility that it is easily broken by only applying a small number of repetitive bending operations. Moreover, in the surface layer portion, the thin-leaf compressed members 203 are in a regular and high orientation state with high density. When the sealing material is bent with a small radius of curvature, therefore, phenomena which reduce the sealing properties, such as cracks or partial flaking easily occur in the thin-leaf compressed members 203 of the surface layer portion. A working process involving a bending operation such as winding or folding is difficult to do. Therefore, there results not only a problem in that products such as a sealing member using the sealing material made of expanded graphite have a narrow application range, but also a problem in that working means are restricted and improvement of product productivity is naturally limited.